(1) Field of the Invention
The present invention relates to a piezoelectric acceleration sensor for detecting acceleration, shock and vibration by using a piezoelectric body, and a method of detecting acceleration, and a manufacturing method thereof.
(2) Description of the Prior Arts
In recent years, dielectrics, particularly ferroelectrics, has been studied as a key material of piezoelectric elements such as an acceleration sensor (piezoelectric acceleration sensor) and a pressure sensor utilzing piezoelectric, an actuator utilizing reverse piezoelectric effect as well as a pyroelectric infrared detector using pyroelectric, a nonvolatile memory using polarization inversion, a capacitive element using a characteristic of high permittivity.
A piezoelectric element used for the above-mentioned acceleration sensor and a pressure sensor detects mechanical values (acceleration, pressure and the like) by utilzing `piezoelectric effect` in which a piezoelectric body generates electric charge by the addition of force to a piezoelectric body, and has a characteristic of obtaining an extremely high detectivity.
The above-mentioned acceleration sensor is described below in detail. Since acceleration, shock and vibration are a similar phenomenon physically, these are chiefly named acceleration generically below.
An acceleration sensor is classified into the following three kinds of types by a direction of detecting a force (acceleration) which acts on a piezoelectric body.
(1) longitudinal effect type: detects an acceleration in a direction parallel with an electrical axis PA1 (2) shear effect type: detects an acceleration in a direction wherein a gap in a plane parallel with an electrical axis occurs PA1 (3) lateral effect type: detects an acceleration in a direction vertical to an electrical axis
The above-mentioned longitudinal effect type acceleration sensor is constituted by providing a weight at the end of a piezoelectric body, and utilizes the occurrence of electric charge in the piezoelectric body when the piezoelectric body expands and contracts by the inertia of the weight. This acceleration sensor can detect a large acceleration because of a high mechanical strength. Moreover, generally, the acceleration sensor can detect a vibration with a high frequency and an acceleration which changes abruptly because of a high mechanical resonance frequency.
A shear effect type acceleration sensor has the same structure as the above-mentioned longitudinal effect type, and utilizes the occurrence of electric charge in a piezoelectric body when shear force acts on the piezoelectric body by the inertia of the weight. This acceleration sensor also can detect a large acceleration as well as a vibration with a high frequency and an acceleration which changes abruptly, similarly to the longitudinal effect type.
Meanwhile, a lateral effect type acceleration sensor is constituted by sticking a piezoelectric body on an elastic plate with the structure of a cantilever or a fixed beam at both ends, and can detect a minute acceleration because of a high sensitivity. Moreover, generally, the acceleration sensor easily detects a vibration with a low frequency and an acceleration which changes gently since it is easy to lower a mechanical resonance frequency.
However, the above-mentioned longitudinal effect type acceleration sensor detects not merely an acceleration in a direction parallel with an electrical axis, but also an acceleration in a direction, wherein a gap in a plane parallel with an electrical axis occurs at a weight, by the same principle as a shear effect type. Consequently, it is difficult to decide through output voltage an acceleration in which direction acts, and the problem is that it is impossible to detect only an acceleration in a direction parallel with an electrical axis accurately. It is possible to detect a large acceleration, while it is difficult to detect a minute acceleration.
Meanwhile, generally, a lateral effect type acceleration sensor has such a problem that it is difficult to detect a vibration with a high frequency and an acceleration which changes abruptly because of a comparatively low mechanical resonance frequency. Since a piezoelectric body is stuck on an elastic plate and an acceleration is detected indirectly according to a deformation of the piezoelectric body following a flexure of the elastic plate, a dispersion in the sensitivity of the sensor tends to get larger when the elastic plate and the piezoelectric body are incompletely stuck in a manufacturing process or a dispersion occurs in a state of sticking.